Typically, a laser oscillator includes a housing in which a pair of discharging electrodes are arranged and a laser medium gas is filled and an optical resonator that is arranged at both sides parallel to the discharging surface of the housing. The housing includes the discharging electrodes as a pair of plate-like electrode materials arranged apart by a predetermined distance, a blower that circulates the laser medium gas inside the housing, and a cooling unit that cools the laser medium gas that has reached a high temperature due to the electric discharge from the discharging electrodes. The gaps between the discharging electrodes and the cooling unit are connected by a duct.
In the optical resonator, a first optical base and a second optical base are arranged parallel to each other by, for example, three supporting rods, two in the upper part and one in the lower part, over both sides parallel to the discharging surface of the housing. A total reflecting mirror is fixed on the first optical base. A partial reflecting mirror is fixed on the second optical base facing the total reflecting mirror. The direction in which the optical resonator resonates a laser light is hereinafter referred to as an optical axis. The gap within a laser light passing portion between the housing and each optical base of the optical resonator is connected by a bellows.
Given below is the description of an outline of the working of such a laser oscillator. When a high voltage is applied to the discharging electrodes, an electric discharge occurs between the discharging electrodes thereby causing an excitation of the laser medium gas between the discharging electrodes. A light generated due to the excitation of the laser medium gas is resonated by resonant mirrors. The laser light reflected at the total reflecting mirror supported on the first optical base reaches the partial reflecting mirror supported on the second optical base. Subsequently, some portion of the laser light is output as it is to the outside of the laser oscillator and the remaining portion of the laser light is reflected toward the total reflecting mirror supported on the first optical base. At that time, the excited laser medium gas between the discharging electrodes passes through the duct and circulates in the cooling unit. After the cooling unit cools the excited laser medium gas, the blower re-circulates the cooled laser medium gas to the discharging electrodes.
In the case of a laser oscillator used for a processing having a high output power, the amount of generated heat is large and a local temperature difference occurs in the circulating laser medium gas, which causes a thermal deformation of the housing. Moreover, the supporting rods of the optical resonator also undergo a thermal deformation following the housing. As a result, the tilt of the optical bases with respect to the supporting rods (positional relation between the pair of optical bases) is changed from the state at the time of installation. That causes a fluctuation in the temporal stability with respect to the output direction of the laser light position. To solve such a problem, conventionally, a configuration has been proposed in which the central portions in the axis direction of two supporting rods arranged in the upper part of a housing from among the supporting rods of an optical oscillator are attached to the top surface of the housing, a supporting rod on a side having less thermal deformation is fixedly attached to the housing, and other supporting rod is movably attached with only the movement in the axis direction and the height direction restricted (for example, see Patent Literature 1).
According to the method disclosed in Patent Literature 1, it is possible to suppress a change in the positional relation between the pair of optical bases when a local temperature difference occurs in the laser medium gas circulating in the housing. However, because the thermal deformation of the housing causes a change in the position or the angle of both end faces, the positions of the bellows arranged on both sides with respect to an optical axis direction of the housing changes. The reaction force generated due to the change of positions of the bellows affects the positional relation between the two optical bases. To solve such a problem, conventionally, a configuration has been proposed in which the side edge portion of the housing and the optical bases are coupled with a leaf spring in such that the housing and the optical bases are restricted from making a curved motion in a direction perpendicular to the optical axis and a rotational motion about the optical axis but allowed to make a curved motion along the axis direction and a rotational motion about the axis in a direction perpendicular to the optical axis (for example, see Patent Literature 2). Taking such a configuration, the alignment change of the optical resonator is suppressed to the minimum.
Meanwhile, in a laser processing apparatus, the laser light output from a laser oscillator is guided to a processing point through a plurality of deflecting mirrors. Typically, the laser light output from a laser oscillator includes a linearly-polarized optical component. The rate of absorption of the laser light with respect to a target object differs according to the processing direction due to the linearly-polarized optical component. That causes an anisotropy in processing quality. Usually, to prevent the anisotropy in processing quality, a conventional technology takes a configuration in which a circularly-polarizing mirror is arranged between the laser oscillator and the deflecting mirrors for converting the laser light emitted by the laser oscillator into a circularly-polarized light (for example, see Patent Literature 3).
To convert the linearly-polarized laser light output from the laser oscillator into a circularly-polarized light, a mirror can be arranged such that, with respect to a reflecting surface of the mirror with an incidence angle of 45° and at an incidence arrangement where a polarization plane of the linearly-polarized laser light makes an angle of 45° (angle of orientation of 45°) with an S-polarized optical axis (or a P-polarized optical axis), a phase difference of 90° (λ/4) occurs between an S-polarized component and a P-polarized component of the reflected laser light. Thus, a dielectric multilayer mirror of optical film design meant to cause the phase difference of 90° (λ/4) between the S-polarized component and the P-polarized component of the reflected laser light is known as the circularly-polarizing mirror. The S-polarized component is a component having a polarization plane perpendicular to an incidence plane and the P-polarized component is a component having a polarization plane perpendicular to the polarization plane of the S-polarized component, i.e., parallel to the incidence plane.
Moreover, regarding a laser processing apparatus, a conventional configuration is also known in which optical components for performing a processing-point check or an optical-path alignment are arranged inside a laser oscillator. In that configuration, the laser oscillator further includes a guide light source that emits a guide light, an optical shutter that blocks the laser light emitted inside an optical resonator at the time of determining stains on or misalignment of a resonant mirror or an external deflecting mirror, and an optical damper that absorbs the laser light reflected at the optical shutter. Moreover, because it is necessary to cool the optical damper or the mirrors, an exit optical path unit made of the optical shutter or the circularly-polarizing mirror has a heavy structure and is fixed to a housing or a mount beneath the housing (for example, see Patent Literature 4). Furthermore, a guide light generating unit constructed of a guide light source and a deflecting mirror is fixed on the outside of a supporting frame in which an oscillator housing including an optical resonator is fixed (for example, see Patent Literature 5).
Patent Literature 1: Japanese Patent Application Laid-open No. 2000-183425
Patent Literature 2: Japanese Patent Application Laid-open No. 2003-304015
Patent Literature 3: Japanese Patent Application Laid-open No. 2002-316291
Patent Literature 4: Japanese Patent Application Laid-open No. H09-271968
Patent Literature 5: Japanese Patent Application Laid-open No. H09-23034